I have black squares being created instead of normal icons on my home screen when new files are created. (Mostly PDF files) When I move the file to another directory (copy or move) they then show with the proper icon, ie PDF icon. Any suggestions?

To Clear and Reset Thumbnail Cache using Disk Cleanup

1. Open Disk Cleanup.

2. Check the Thumbnails box, and click/tap on OK. (see screenshot below)
Name:  Clear1.jpg
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Size:  72.9 KB

3. Click/tap on the Delete Files button. (See screenshot below)

Name:  Clear2.jpg
Views: 118202
Size:  17.7 KB

4. Disk Cleanup will now clear the thumbnail cache and close.